1. Field of the Invention
The present invention relates to ZnO/sapphire substrates used for surface acoustic wave devices, various optical devices, semiconductor devices, and the like. In particular, the present invention relates to a ZnO/sapphire substrate in which a (1 1-2 0) ZnO film is deposited on an R-plane sapphire substrate whose (0 1-1 2) planes are parallel to the surface thereof and to a method for manufacturing the substrate.
2. Description of the Related Art
Various piezoelectric materials are used as substrates of surface acoustic wave devices and the like. A paper in the Journal of Crystal Growth 45 (1978), 346-349 discloses a structure in which (1 1-2 0) planes of a ZnO single-crystal film are disposed on the (0 1-1 2) planes of a sapphire substrate by magnetron sputtering.
The R-plane sapphire readily allows a ZnO single-crystal film to epitaxially grow thereon under conditions which are not strict. However, the inventors found that when (1 1-2 0) planes of ZnO are epitaxially disposed to form a ZnO film on the R-plane sapphire, a residual stress occurs in the ZnO film and it often causes cracks in the ZnO film.
Accordingly, an object of the resent invention is to provide a high-quality ZnO/sapphire substrate in which ZnO (1 1-2 0) planes are disposed to form a ZnO film on an R-plane sapphire substrate whose (0 1-1 2) planes are parallel to the surface thereof. The ZnO/sapphire substrate does not exhibit cracks, and is therefore capable of being used for various devices. Also, an object of the present invention is to provide a method for manufacturing the ZnO/sapphire substrate.
The inventors have been considered that the residual stress in the ZnO film, which is caused by a mismatch between the lattice constants of ZnO crystals and sapphire crystals, causes cracks in the ZnO film, and have conducted intensive research to accomplish the objects.
According to one aspect of the present invention, there is provided a ZnO/sapphire substrate comprising an R-plane sapphire substrate whose (0 1-1 2) crystal planes are parallel to the surface thereof and a ZnO epitaxial film formed on the R-plane sapphire substrate. The (1 1-2 0) crystal planes of the ZnO epitaxial film are parallel to the (0 1-1 2) crystal planes of the R-plane sapphire substrate, and an interplanar spacing of (1 1-2 0) crystal planes of the ZnO epitaxial film is set in a range of about 1.623 xc3x85 to 1.627 xc3x85.
According to another aspect of the present invention, a method for manufacturing a ZnO/sapphire substrate is provided. The method comprises the steps of: preparing an R-plane sapphire substrate whose (0 1-1 2) crystal planes are parallel to the surface thereof; and epitaxially growing a ZnO film on the R-plane sapphire substrate such that (1 1-2 0) crystal planes of the ZnO epitaxial film are parallel to the (0 1-1 2) crystal planes of the R-plane sapphire substrate, and setting the interplanar spacing of (1 1-2 0) crystal planes of the ZnO epitaxial film in a range of about 1.623 xc3x85 to 1.627 xc3x85.
The present invention is also directed to a surface acoustic wave device comprising the ZnO/sapphire substrate and an interdigital transducer formed on the ZnO/sapphire substrate.